1. Field of the Invention
The present invention relates to a medical imaging apparatus and medical imaging method for imaging in a state that a contrast agent is injected into a subject.
2. Description of the Related Art
An example of a method for locating a lesion in a blood vessel or a lesion in an organ around a blood vessel is CTA (Computed Tomography Angiography). Such a lesion is aneurysm, vascular occlusion, or the like. An image obtained by a general X-ray CT (Computed Tomography) apparatus is low-contrast because the X-ray absorption value in a soft tissue such as an organ and a blood vessel is small in the image. Therefore, it is difficult to make a sufficient diagnosis of the soft tissue by the image obtained by the general X-ray CT apparatus. In contrast, CTA is a method of injecting a contrast agent with a high X-ray absorption value into a subject and imaging in a state that the X-ray absorption value in a focused blood vessel is increased. By employing CTA, it is possible to increase contrast and obtain a clear image of the organ.
There is a technique of determining the timing to start imaging by an X-ray CT apparatus in accordance with the concentration of a contrast agent injected into a subject (refer to Japanese Unexamined Patent Application Publication No. 2007-143880). In this technique, the contrast agent is injected into the subject by a contrast agent injector, and the concentration of the contrast agent in a region of interest (ROI) is monitored. When the concentration of the contrast agent in the ROI exceeds a threshold, the X-ray CT apparatus automatically starts imaging. This technique enables an operator to cause the imaging apparatus to determine the timing of imaging in accordance with the concentration of the contrast agent and to automatically start imaging, only by giving an instruction to start imaging to the imaging apparatus.
With reference to FIG. 1, the timing to start imaging according to a conventional technique will be described. FIG. 1 is a view for describing the timing to start imaging, and is a graph showing change in concentration of a contrast agent in an ROI of a subject.
The contrast agent injected into the subject moves in the subject on a blood flow and reaches a target organ. In imaging by the X-ray CT apparatus, the timing of flow of the contrast agent into the target organ is determined to start a scan (may be referred to as the “main scan” hereinafter) and acquire image data.
In the conventional technique, a scan called a prep scan may be executed before the main scan. The prep scan is a scan for observing change in concentration of the contrast agent in the ROI of the subject with the contrast agent injected. The prep scan is a scan method of performing image reconstruction while collecting X-ray projection data of the subject, and generating image data in real time while scanning.
The concentration of the contrast agent in the ROI is obtained based on the image data obtained by the prep scan and, at a point that the concentration of the contrast agent is increased to a certain extent, the prep scan is stopped and the main scan is automatically started. In this case, when the CT value (the concentration of the contrast agent) within the ROI becomes a threshold or more, the main scan is automatically started.
A curve showing temporal change in stain of a contrast agent is called a Time-Density Curve (referred to as the “TDC” hereinafter).
For example, a curve showing temporal change in CT value within the ROI shall be the TDC. When the contrast agent flows into a tissue, the CT value in the tissue is increased. Thus, the CT value represents the state of stain of the contrast agent.
FIG. 1 shows an example of the TDC. The horizontal axis takes time t, and the vertical axis takes CT values (pixel values). At a point immediately after injection of the contrast agent into the subject, the contrast agent is not yet flown into the ROI. Therefore, as shown by a TDC 500, the CT value within the ROI is low. After a lapse of time, the contrast agent flows into the ROI and the CT value changes. For example, the CT value increases. When the CT value in the ROI becomes a predetermined threshold or more, it is assumed that the contrast agent already flows into a target organ. Therefore, the main scan is started at this point. Since the CT value in the ROI becomes the threshold or more at a time T0, the main scan is started at the time T0.
Thus, the main scan is automatically started, and it becomes possible to start imaging at the timing that the contrast agent flows into the target organ. Then, by executing the main scan for a certain time having been set in advance, CT image data representing the target organ with the contrast agent injected is acquired. The main scan is executed from the time T0 that the main scan starts to a time T1 after a lapse of the certain time, and the main scan is ended at the time T1.
However, in the method according to the conventional technique, only the timing to start injection of the contrast agent and the timing to start the main scan are controlled. In the conventional technique, during the main scan, the injection amount and injection pressure (injection speed) of the contrast agent are not controlled based on the TDC. Moreover, in the conventional technique, control of ending injection of the contrast agent in response to end of the main scan is not performed. For example, as shown in FIG. 1, injection of the contrast agent is still continued after the time T1 that the main scan ends. Since injection of the contrast agent into the subject is continued though the main scan ends, the contrast agent is wastefully injected, and a burden is imposed on the patient.
The change in concentration of the contrast agent (the temporal change in CT value) largely varies depending on the weights of patients, diseases, and so on. To be specific, a time from start of injection of the contrast agent to flow of the contrast agent into the target organ varies depending on patients and the types of diseases. In the conventional technique, the injection amount and injection pressure of the contrast agent are not controlled during the main scan. Therefore, even when the main scan is started at the time T0, the main scan may end before the contrast effect increases (the CT value increases), depending on patients and the types of diseases. In this case, the main scan cannot be executed with high contrast effect, and it is therefore difficult to acquire image data having a desired contrast. Since there is a need to perform imaging again in such a case, a burden is imposed on the patient.